US2966514A - Method of producing benzene polycarboxylic acid - Google Patents

Description

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METHOD OF PRODUCING BENZENE POLY- C r OXYLIC ACID Anton Benning, Werner Emte, and Otto Grossldnsliy, Dortmund-Eving, and Heinrich Fruhbuss, Munich, Germany, assignors to Bergwerksverband G.rn.h.H., Essen, Germany No Drawing. Filed June 14, 1955, Self. No. 515,561 Claims priority, application Germany Feb. 25, 1954 Claims. (Cl. 260-524) derivatives containing both chloromethyl and alkyl substituents in the benzene ring by means of a cheap oxidizing agent such as nitric acid does not result in completely oxidized products, e.g. in benzene polycarboxylic acids wherein carboxyl groups are the only substituents in the benzene ring. Attempts to carry out such oxida tion with nitric acid at atmospheric pressure did not result in completely oxidized products, while attempts to carry out the oxidation with nitric acid at superatmospheric pressure resulted in poor yields of acids contaminated to a great extent with chlorinated and nitrated acids which are extremely difiicult to separate from the desired product.

It is accordingly a primary object of the present invention to provide a method of oxidizing benzene derivatives containing both chloromethyl and alkyl groups as substituents in the benzene ring by means of nitric acid in a simple and inexpensive manner whereby high yields of easily isolatable and uncontaminated benzene carboxylic acids are obtained without the formation of considerable amounts of byproducts.

It is another object of the present invention to provide a method of oxidizing chloromethylated alkyl benzenes with nitric acid whereby the nitric acid utilized is regenerated and returned to the oxidizing process.

It is still another object of the present invention to provide a method of producing terephhalic acid in good yield by the oxidation of p-chloromethyl toluene.

It is yet another object of the present invention to provide a method of producing high grade pyromellitic and trimellitic acid from cheap crude xylene.

It is a further object of the present invention to provide a method of producting mixtures of trimellitic and pyromellitic acid.

It is still a further object of the present invention to provide a method of producing benzene carboxylic acids containing any number of carboxyl groups and wherein the carboxyl groups may be placed in the molecule in any desired position.

Other objects and advantages of the present invention will be apparent from a further reading of the specification and of the appended claims.

With the above objects in view, the present invention mainly consists in a method of producing benzene polycarboxylic acids, comprising the steps of heating a benzene derivative consisting of a benzene ring substituted 2,9524 Patented Dec. 27, 1560 with at least one alkyl group and with at least one chloromethyl group with nitric acid at substantially atmospheric pressure at a temperature sufiiciently high to oxidize the chloromethyl group until a substantially chlorine-free reaction product is formed, thereby converting the chloromethyl group to a carboxyl group without substantially affecting the alkyl group and thus forming essentially an alkyl benzene carboxylic acid, and heating the chlorine-free reaction product with nitric acid under superatmospheric pressure at a temperature sutficiently high to oxidize the alkyl group, thereby converting the alkyl group to a carboxyl group and thus obtaining a benzene carboxylic acid consisting of a benzene ring substituted only with carboxyl groups.

The starting materials for the method of the present invention are chloromethylated alkyl benzene derivatives such as toluene, Xylenes, ethylbenzene, mesitylene, durol, and the like or mixtures thereof. By the chloromethylation of these alkyl benzene derivatives, which per se is known, one or two hydrogen atoms of the benzene nucleus are replaced by chloromethyl groups (-CH Cl), thus producing benzene derivatives containing one or two chloromethyl groups in addition to the alkyl group or groups, e.g. CH C H etc., already present in the original alkyl benzene derivative.

In practice the chloromethylation is carried out by treating alkyl benzenes with chloromethyl ether or hydrogen chloride and formaldehyde (or its polymerides), whereby, depending on the reaction conditions, one or two chloromethyl groups are incorporated into the benzene ring. Thus, for example, toluene can be converted into a derivative represented by the formula C H .CH (CH Cl) xylene is analogously converted into 0r C6H2(CH3)2 (CH2Cl)2, etc. Subsequently, the chloromethylated benzene homologues of the kind aforesaid are subjected to the twostage oxidation treatment of the present invention, the first stage being carried out at substantially atmospheric pressure to convert the chloromethyl group or groups to a carboxyl group or groups, and the second stage being carried out at superatmospheric pressure to convert the non-converted alkyl group or groups to a carboxyl group or groups. Thus, the number of carboxyl groups in the final benzene carboxylic acid is equal to the number of alkyl groups plus the number of chloromethyl groups present in the original benzene derivative.

According to a preferred embodiment of the present invention p-chloromethyltoluene is oxidized in the above described two-stage oxidation method to produce substantially pure terephthalic acid.

According to another embodiment of the present invention the total number of substituents convertible to carboxyl groups by oxidation, i.e. the total number of alkyl and chloromethyl groups is three, whereby a henzene tricarboxylic acid, such as trimellitic acid, is formed. The total number of three oxidizable groups may be reached by one chloromethyl group and two alkyl groups, or two chloromethyl groups and one alkyl group.

In accordance with the present invention the total number of alkyl and chloromethyl groups may be four, five or six whereby the resulting benzene carboxylic acid will be respectively pyromellitic acid, benzene pentacarboxylic acid or mellitic acid.

Preferably the alkyl groups of the alkyl benzene are methyl groups, e.g. as in toluene, xylenes, mesitylene, durol and the like.

It has been found according to the present invention that only by the above described two-stage oxidizing treatment is it possible to obtain pure benzene polycarboxylic acids in excellent yield, with all of the substituents having been oxidized to carboxyl groups. In view of the useless attempts made prior to the present invention to achieve a satisfactory and complete oxidation at elevated pressure, the new method of the present invention is quite surprising as to the excellent yields and simplicity of proceeding. Prior to the present invention complete oxidation could only be achieved by means of expensive potassium permanganate or expensive roundabout procedures. The process of the present invention allows for the production of all the benzene carboxylic acids, including the trivalent acids and their derivatives up to mellitic acid, starting almost and in the most interesting cases from readily obtainable benzene derivatives, especially toluene, and pure and crude xylenes. Moreover, the process is extra-ordinarily simple in its performance, as will be seen from the following description.

The most suitable temperature for the heating during the first stage is the boiling point of the nitric acid, about 105 C. The first stage should be carried out at about atmospheric pressure although a slight excess pressure may result in the speeding of the reaction in the first stage without adversely affecting the entire reaction.

During the second stage the superatmospheric pressure should be at least 3 atmospheres, preferably between 3 and 30 atmospheres and most preferably between 5 and 20 atmospheres. A pressure of about 5 atmospheres gives highly satisfactory results although the reaction proceeds faster at a pressure of about 15 atmospheres and at a temperature of about 150200 C.

The method can be carried out in practice in a single reactor, so that isolation of the intermediate product is unnecessary and the operating conditions only need be intensified as soon as all the chlorine has been separated from the starting material. This clearly necessitates a reactor which is resistant to nitric acid and hydrochloric acid, e.g. such as one of nickel alloyed with 16-18% of molybdenum, l5.5-l7.5% of chromium, 4.5-7% of iron, 3.75-4.75% of tungsten and small amounts of silicon or manganese.

Alternatively, the difiicultly soluble crude acid of the first reaction stage can be separated and further oxidized in a second reactor. Isolation of the intermediate product and further oxidation in a second reactor have various advantages. The intermediate product can be freed by washing with a suitable alkyl benzene, e.g. toluene, from the last traces of impurities, such as non-converted chloromethyl alkyl benzene, and so be in the best form for obtaining an end product which fulfills the highest requirements of purity. The oxidation in two separate reactors is also of advantage since the corrosive action of the reaction mixture is very diiferent in the two stages. It has been found that it is most advantageous to carry out the first stage in a reactor lined with a material fully resistant to chlorine and chlorine compounds, e.g. in ceramic reactors. For carrying out the second oxidation stage, special autoclaves of alloy steel are suitable, such as those made of nickel, chromium, molybdenum, tungsten, titanium or their alloys or covered with such materials.

It is clearly not necessary in operating in vessels of different material to cool the reaction mixture of the first stage and separate the product. If a saving in autoclaves is desired, in which the second oxidation is to be carried out, the last traces of chlorine are removed from the intermediate product, this being achieved most safely by separating and washing the crude acid with liquid alkyl benzenes, such as toluene, xylene, and the like. Said alkyl benzenes are especially suitable for washing, offering the chance of being used further as starting materials for the production of further amounts of benzene carboxylic acids after enrichment with chlorine compounds according to the process of the invention, needing no purification, e.g. by distillation, for the purpose of re-employment as washing solvent.

The oxidation of chloromethyl derivatives can be carried out equally well, as mentioned, in the same autoclave, merely by raising the pressure and temperature on completion of the first oxidation reaction. Such a process is particularly satisfactory since, in the first oxidation stage, a stronger nitric acid than in the pressure stage leads to the best results and the decrease in the ntiric acid concentration after the first oxidation stage leads to a suitable acid concentration for the second stage.

It has been found that the oxidation proceeds rapidly and Without the disadvantage of obtaining final products contaminated by nitro compounds or other by-products, by the utilization of a relatively highly concentrated nitric acid during the first stage of oxidation at substantially normal pressure and utilizing a relatively weaker nitric acid concentration during the second stage of oxidation under superatmospheric pressure.

The best results are obtained utilizing nitric acid of about 30-47% concentration in the first stage and about 20-45% concentration in the second stage. Preferably the concentration of nitric acid in the first stage is 38- 47% and most preferably, the concentration of nitric acid in the first stage is about 43-45%. Preferably, the concentration of nitric acid in the second stage is between 3038% when operating with a benzene derivative containing at least three constituents oxidizable to carboxyl groups and about 32-35% when starting with chloromethyltoluene.

Despite the difference between the most advantageous concentrations of the nitric acid in the first and second stages it is possible to subject the entire reaction mixture of the first oxidation stage without interruption to the further oxidation carried out under pressure. It has proved that in practice the concentration of the nitric acid in the first stage decreases accurately during the oxidation so as to attain the most suitable concentration for the second stage. It goes without saying that within the scope of the process of the invention it is possible also to work with stronger or weaker nitric acids in both stages. As a rule, however, the concentrations mentioned above give the best results.

When working with a nitric acid concentration of about 38-47%, a rather chlorine-free intermediate product is obtained in the first stage. The absence of chlorine is important for the quantitative course of the subsequent oxidation as well as for saving the pressureapparatus. It is obvious that hydrogen chloride can seriously damage the apparatus, especially under elevated pressure. For carrying out the second operating stage, apparatus of special steel resistant to compression and nitric acid are suitable, such as those made of nickel, chromium, tungsten, titanium, molybdenum or their alloys, or, at least apparatus provided with equipments and internal coatings consisting of such materials. For the first oxidation stage and the chloromethylation it is recommendable to operate in reactors made of material resistant to hydrochloric acid, e.g. in ceramic or enamelled reactors which may be less resistant to pressure, however.

The consumption of the acid is known to be of great importance to the economy of a nitric acid oxidation, and is caused by the reduction of the acid to elementary nitrogen. With nitrogen, nitrous gases are separated out in such oxidation, which can be readily recovered by washing. However, the nitric acid reduced to nitrogen is lost for an oxidation method.

It has now been found that in both oxidation stages the reduction of nitric acid to nitrogen can be substantially completely supressed without affecting or slowing the reaction. For this purpose, oxygen or air is passed through the reaction mixture during the reaction. The expected frequent explosions do not occur. The gas is introduced in finely-divided form from below into the reactor, so that it rises through the reaction mixture in the form of small bubbles. The nitric acid in this mode of operation is reduced substantially only to nitrogen dioxide, which can be extracted and returned to the reactor if required. In this way, the method proceeds without significant losses of nitric acid, so that the oxygen required for the oxidation is supplied substantially exclusively by the gases introduced. The reaction proceeds faster and with the best yields, since too great a lowering of the nitric acid concentration during the reaction is avoided. If operating instead without simulaneous introduction of oxygen, the second oxidation stage must be carried out with higher pressures and temperatures in order to achieve approximately equivalent results.

In the oxidation of the chloromethylated alkyl benzene in the first stage, not only is nitric oxide evolved from the reaction mixture, but also hydrogen chloride, which can likewise be recovered. For this purpose, the gas is passed for instance through concentrated sulfuric acid. Nitrosylsulfuric acid is formed from which nitric acid can be readily obtained, while the hydrogen chloride gas evolves from the sulfuric acid and can be used again for preparing chloromethylated derivatives.

A vertical heatable reaction tube is particularly suitable for continuous oxidation, in which the reaction components are introduced from below and the gas through a frit or the like. The reaction mixture rises in the tube according to the amount of fresh starting material supplied, without too great an intennixing of the reaction mixture occurring along the tube. The supply of new starting material is controlled so that reacted mixture always extends to the upper end of the tube. The mixture can be transferred to a container in which the desired acid separates. From the container, which can be cooled if required, the mother liquor flows over into a cooling tube. Together with the mother liquor, the nitrous gases flow downwards through the tube, wherein all the nitrous gases are surprisingly taken up again by the mother liquor, so that at the foot of the cooling tube nitric acid is obtained which is fully utilizable for the method and can be returned to the reactor if necessary. That all the nitrous gases are taken up by the nitric acid is shown since in the waste gas which is blown from the container through a valve from the enclosed system, substantially no nitrous gases remain.

As aforesaid, the oxidation can be carried out without interruption merely by changing the operating conditions in a single apparatus. This, however, necessitates an apparatus resistant to hydrochloric acid and to nitric acid or mixtures thereof, respectively. For the reasons pointed out already it is preferable, however, to change the apparatus. In this case it may be very advantageous to separate and to wash the intermediate oxidation product. This measure is especially recommendable when obtaining intermediate products difficultly soluble in nitric acid, since the separating does not cause any difficulty, resulting more safely in chlorine-free intermediate products.

The chloromethylation preceeding the oxidation of the benzene derivatives suitable as starting materials for the however, to produce preferably one of the two chloro-' methyl compounds. In general, the chlorornethylation' of e.g. m-xylene results in 4-chloromethyl-l,3-dimethyl-' benzene and 4,6-bis-chlorornethyl-1,3-dimethylbenzene. This mixture is obtained for example by treating m-xylene at about -70 C. with four to five times its weight of hydrochloric acid and about the same amount by Weight of formaldehyde with introduction of hydrogen chloride and vigorously agitating. When formaldehyde is present in larger excess, the bis-compound is preferably obtained. The non-reacted formaldehyde, paraformaldehyde etc. is not lost but is completely consumed by repeated employment of the reaction solutions. If bis-compounds are to be produced exclusively, the mono-compounds are separated and subjected to another chloromethylation. In this way trimellitic acid or other benzene-tricarboxylic acids are obtained from toluene via the bischlo-romethyl-toluene as intermediate products, and pyromellitic acid is obtained from xylene via the bis-intermediate products mentioned above.

A special advantage of the process of the invention is the fact that the processing of practically all the xylene isomers leads exclusively to trimellitic and/ or pyromellitic acid. Consequently, the new process offers the chance of producing these high-grade acids from the so-called crude xylenes. According to their origin these crude xylenes represent a varying mixture of 0-, mand p-xylene. The crude xylene of coke oven plants contains for example about of m-xylene, 20% of p-xylene, 5% of o-xylene, and, in addition, about 5% of ethylbenzene. The crude xylenes of the petroleum industry consist of isomers, too. Separating the single isomers is diflicult due to their boiling points being closely together.

Consequently, the single pure isomers represent comparatively expensive starting materials; on the other hand the crude xylenes are very cheap products. Thus the considerable importance of the process starting from any arbitrary crude xylene and resulting in pure trior pyromellitic acid, is self-explanatory.

It may be additionally mentioned that by the process of the invention inferior amounts of hemimellitic and 1,2,3,4-benzene-tetra-carboxylic-acid are formed. Both acids can be readily separated from trimellitic and pyromellitic acid and partially do not crystallize at all out of the oxidation solution together with the latter acids. As they result only in small amounts, as pointed out, a separation and isolation of these acids may be generally dispsened with.

To which extent almost exclusively triand pyromellitic acid result when working up pure and crude xylenes may become apparent by the following equations surnming up the process of the chloromethylation of xylenes, the products obtained making obvious the result of the subsequent oxidation.

CHzCl C|1H2Cl -CH3 CH3 CH3 ClHzC- Ha CH CH3 CH3 (7H .(llHa CHgCl CHzCl l ClHsC Ha CH; CH:

(2H3 (11H: (3H (|JH CH:

CH CH3 -CH3 CH3 CH;

CHiCl ClI-I2C CHgCl CHzCl 2C1 HzCl l oxidation l (30 OH ('30 OH O O OH -O O OH I H000- 30 OH 470 OH In addition to crude xylenes as starting materials for the production of benzene polycarboxylic acids, mainly toluene, single xylene isomers, triand higher methylbenzenes and the like are suitable, too. Derivatives of these compounds, such as nitro-derivatives and all the mixtures of the starting materials concerned are suitable also, certainly all the benzene derivatives or their mixtures which, after having been chloromethylated, contain substantially at least 3 substituents oxidizable to carboxyl groups.

When starting from a crude xylene, the mixture mentioned already and consisting of monoand bis-chloromethylxylene is obtained by chloromethylation, representing already a commercially valuable mixture of acids consisting substantially of pyromellitic acid, in addition to trimellitic acid after having been oxidized according to the process of the invention. Instead of this, the mono-product (boiling range 105l25 C.) can be separated by distillation from the mixture of the monoand bis-chloromethylated products, and the distillate and the residue can be oxidized separately, thus yielding trimellitic acid as well as pyromellitic acid either or both in isolated condition.

Furthermore, it has been found that in producing chloromethylated products a separate and partial isolation of monoand bis-products is entirely possible. It has been found that when cooling the reaction solution a part of the bis-compounds crystallizes out already and can therefore be separated. By stronger cooling it is even possible to separate the greater part of the bis-compounds by simple means and subsequently oxidize them separately. Consequently, the remaining solution can be also removed separately or worked up as described above. Hereby the distillation residue is oxidized together also with the part separated already, that is together with ali the bis-compounds. Thus, there is the chance to adjust the course of the process to the respective requirement of final products. The conditions illustrated for xylene are valid analogously for all other suitable starting materials.

When working up xylenes and crude xylenes, it has been found that further possibilities of obtaining single benzene carboxylic acids instead of a mixture of acids are available. It has been found that decomposing the chloromethylated product into well defined fractions and subjecting these fractions to separate oxidation is not absolutely necessary. The acids resulting during the oxidation of the entire chloromethylated product, such as pyromellitic acid and trimellitic acid possess quite different properties as to solubility and consequently crystallize out under completely different conditions. When cooling the oxidation Solution, pyromellitic acid precipitates quantitatively without simultaneous precipitation of the l,2,3,4-bcnzene-tetra-carboxylic-acid containing also four carboxyl groups, and can be readily separated from the oxidation solution. Trimellitic acid is highly soluble even in cold water, being not obtained therefore prior to evaporating the oxidation solution. If required, it can be readily freed from contaminations by l,2,3,4-benzene-tetra-carboxylic-acid and hemimellitic acid by sublimation or a similar method.

In carrying out the process in practice, it has surprisingly been found when processing crude xylene, that terephthalic acid results as by-product in excellent purity and considerable amounts. It is known to be difiicultly soluble even in hot nitric acid and consequently can be readily separated from the hot oxidation solution of the pyromellitic acid crystallizing out not before being cooled. Instead of this, both acids can be first separated together from the cold oxidation solution and boiled with water whereby pyromellitic acid is dissolved, unlike terephtalic acid, which is then filtered off. The filtrate is cooled, the pyromellitic acid crystallizing out as the hydrate in pure condition.

On the other hand, when separating the mono-compound from the chloromethylated product of a crude xylene and separately oxidizing said compound, the terephthalic acid is obtained from the oxidation solution in this very stage of the process in addition to trimellitic acid.

The terephthalic acid is formed from ethylbenzene contained in the crude xylene. Thus, the surprising purity of this acid is simultaneously explained, since the ethyl group can be oxidized more readily than the methyl group. The formation of the less desired toluic acid resulting readily as by-product when being produced from p-xylene, is prevented accordingly.

Nevertheless it is surprising Without any doubt that the ethylbenzene contained in commercial xylene is almost completely chloromethylated in para-position under the usual conditions of chloromethylation, terephthalic acid being practically obtained according to the amounts of ethylbenzene contained in the crude xylene.

The following examples are given to further illustrate the process of the present invention, the scope of the invention not however being limited to the specific details of the examples. The benzene polycarboxylic acids obtained in the examples are perfectly white so that they can be used directly for preparing resins, lacquers or plasticizers.

Example I 100 parts of m-xylene, 200 parts of aqueous formaldehyde solution (30-40%) and 400 parts of concentrated hydrochloric acid are heated for six hours to 75% C. After adding another 200 parts of formaldehyde solution, the reaction mixture is heated to about 70 C. with introduction of hydrogen chloride for an additional ten hours.

The reaction solution is allowed to cool and the resulting oily crystal layer is separated. The crystals contained therein are filtered. parts of 4-6-bis-chloromethyl-1.3-dimethyl-benzene (melting point 98 C.) are,

obtained. The liquid reaction products are washed with water. The yield is 95 parts of chloromethylated products (dry weight).

The crystalline bis-chloromethyl compound (75 parts) is heated to boiling with 750 parts of 40% nitric acid in a vessel having a ceramic coating, for two hours with introduction of oxygen. The resulting insoluble acid is filtered off and the filtrate-4f necessary after replenishing the nitric acid-is used for oxidizing other chloromethylated products. The carboxylic acid obtained is oxidized with 500 parts of 30% nitric acid for one hour at 170 C. under atmospheres pressure while simultaneously introducing air. 75 parts of pyromellitic acid are obtained which crystallize out of the oxidation solution upon cooling.

95 parts of the remaining chloromethylated product are heated for two hours at 105-ll0 C. with 900 parts of 45% nitric acid while introducing air. The hot emulsion is pumped into a stainless steel reactor and oxidized for one hour at 175 C. under 12 atmospheres pressure, air being passed through simultaneously. Upon cooling the oxidation solution, 50 parts of pyromellitic acid crystallize out yielding an acid of the acid number 880 and the melting point 270275 C. upon being recrystallized from water. The nitrous acid reaction solution is evaporated under reduced pressure until the crystallization starts, and the nitric acid is recovered. 40 parts of trimellitic acid are obtained. I

Consequently the yield according to this manner of operating amounts to 125 parts of pyromellitic acid and 40 parts of trimellitic acid.

For comparison, the aforesaid 75 parts of crystalline bis-chloromethylated product are directly treated at 170 C. and 10 atmospheres of pressure with 750 parts of 32% nitric acid. A crude acid is obtained containing 28 parts of di-methyl-benezene-di-carboxylic acid plus 43 parts of pyromellitic acid.

Example II 100 parts of m-xylene are chloromethylated as described in Example I, but after the separation of the crystals (75 parts) the liquid fraction of the chloromethylated products is distilled off. At a pressure of 14 mm. Hg and a temperature of 100110 C. 45 parts of 4-chloromethyl-1,S-dimethylbenzene are obtained as distillate, while the distillation residue becomes crystalline upon being cooled, consisting of 50 parts of bischloromethylxylene.

The aforesaid 45 parts of mono-chloromethylxylene are oxidized with 400 parts of nitric acid (density 1.27) for two hours at l05l10 C. with introduction of air. The acids obtained are filtered off and oxidized with 300 parts of nitric acid (density 1.20) for one hour at 165 C. and under 15 atmospheres of pressure, air being passed through the reaction mass. After evaporating the nitric acid, 45 parts of trimellitic acid are obtained.

If, however, the 45 parts of mono-chloromethylxylene are returned to the chloromethylation process, 58 parts of the bis-chloromethyl-compound are obtained there from. Consequently 75+50+58=183 parts are available, yielding, when oxidized in the manner described with reference to the mono-product and carried out in the same proportion of ingredients, 185 parts of pyromellitic acid.

Thus, Example 11 shows the method of producing from 100 parts of m-xylene either 185 parts of pyromellitic acid or 45 parts of trimellitic acid and 130 parts of pyromellitic acid, said 130 parts of pyromellitic acid resulting by oxidizing the aforesaid (75+50) parts of the bis-chloromethyl-compound.

Example II.

100 parts of crude xylene are heated for ten hours to 10 tion (30-40%) and 500 parts of concentrated hydro chloric acid, introducing hydrogen chloride. The xylene is not'added until the aqueous formaldehyde-hydrochloric-acid-solution is saturated with hydrogen chlo ride. After the reaction is completed the organic phase is separated still in hot condition, so that the bis-chloro-' methyl-xylenes cannot crystallize out. For this reason the washing with water is done also in hot condition. 160 parts of chloromethylated products are obtained.

These products are heated to boiling for two hours with 1500 parts of nitric acid (density 1.26) with introduction of air. The mixture of acids obtained is filtered off and oxidized with 1000 parts of 35% nitric acid for one hour at 175 C. under 10 atmoshperes of pressure, air being passed in, too. After evaporating the nitrous acid reaction solution, 160 parts of a mixture of acids are obtained consisting of pyromellitic, trimellitic, terephthalic acid and small amounts of 1,2,3,4-benzene-tetra-- carboxylic-acid, hemimellitic and phthalic acid.

However, when directly filtering oit the insoluble substances (still in hot condition) from the resulting nitric acid oxidation solution, 9 parts of terephthalic acid are obtained. After being washed with hot water, the acid number of the terephthalic acid is 674. When cooling the hot terephthalic acid filtrate (oxidation solution and Wash-water), 109 parts of pyromellitic acid crystallize out, which are recrystallized in Water, the acid number being 880 and the melting point 269273 C. after drying. Then the nitrous reaction solution is evaporated and the remaining acids are recovered. 35 parts of a mixture of acids are obtained consisting substantially of trimellitic acid.

Example IV parts of crude xylene are chloromethylated as described in Example III, parts of chloromethylated products being obtained.

160 parts of the chloromethylated product are heated for two hours at l051l0 C. with 1500 parts of nitric acid (density 1.28) with introduction of air. The mixture of acids obtained is filtered off and oxidized with 1000 parts of 33% nitric acid for one hour at C. under 15 atmospheres of pressure, air being passed through the reaction mass. The oxidation being complate, the solution is allowed to cool and the insoluble substance is filtered 01f. The filtrate is evaporated, thus recovering the nitric acid. The residue of the evaporation is recrystallized in dilute hydrochloric acid, yielding 35 parts of trimellitic acid. The filtered reaction solution can be used also to oxidize fresh substance, thus serving to enrich the trimellitic acid and other readily soluble carboxylic acids, such as 1,2,3,4-benzene-tetracarboxylic-acid.

The part precipitated upon cooling the oxidation solution is boiled with water and filtered in hot condition. 112 parts of pure pyromellitic acid crystallize as a hydrate out or" the filtrate. The substance insoluble in water (10 parts) consists of pure terephthalic acid.

Example V 100 parts of crude xylene are treated with formaldehyde and hydrochloric acid as indicated in Example I. The chloromethylation having been completed, the mixture is allowed to cool and the oily crystal layer is separated from the aqueous reaction solution. The crystals are filtered off from the oily crystal layer and eventually recrystallized in petroleum ether. The liquid chloromethylated products are Washed with water and acid (density 1.28), air being passed through the reaction solution. The oxidation is carried out in a ceramic-coated vessel. The hot oxidation solution is pumped into a stainless steel reactor and oxidized for one hour at 170 under atmospheres pressure, with introduction of air. The insoluble part is filtered off and washed with hot water. 11 parts of terephthalic acid (acid number 674) are obtained.

Upon evaporating the filtrates, 39 parts of trimellitic acid crystallize out. The mother liquids are collected and the remaining benzene carboxylic acids are recovered from them.

Example VI 100 parts of 2,4-bis-chloromethyl-toluene are heated for two hours to boiling with 850 parts of nitric acid (density 1.28) with introduction of air. The resulting crude acid is washed with toluene. A chlorine-free methylphthalic acid is obtained which is oxidized with 600 parts of 32% nitric acid for one hour at 165l70 C. under 10 atmospheres pressure, air being passed simultaneously through the oxidation solution. After evaporating the nitric acid, 105 parts of trimellitic acid are obtained.

Example VII 100 parts of pseudocumene (1,2,4-trimethylbenzene) are heated for 3 hours to 70 C. with 200 parts of aqueous formaldehyde solution (3040%) and 500 parts of concentrated hydrochloric acid. After adding 100 parts of aqeuous formaldehyde solution, the mixture is heated for another 7 hours to 70 C. with introduction of hydrogen chloride. The reaction solution is allowed to cool and the crystals are separated. Eventuallythey can be purified by re-crystallization in petroleum ether. 140 parts of bis-chloromethyl-tri-methyl-benzene (melting point 84 C.) are obtained.

140 parts of the bis-chloromethyl-compound are heated to boiling for 2 hours with 1200 parts of nitric acid (density 1.275) with introduction of air. The acid obtained is filtered off and oxidized for one hour at 180 C. under atmospheres pressure with 900 parts of 35% nitric acid. The oxidation solution is evaporated until crystallization starts, thus the nitric acid being recovered. 135 parts of benzene-penta-carboxylic acid (melting point 233-235 C.) are obtained.

Example VIII 100 parts of durene (or isodurene) are heated for 8 hours to 75 C. with 300 parts of aqueous formaldehyde solution (30-40%) and 400 parts of concentrated hydrochloric acid, additionally introducing hydrogen chloride. The oily reaction layer is separated and cooled. The crystals obtained are filtered off and the liquid chloromethyl fraction is returned to the chloromethylation. 90 parts of bis-chloromethyl-durol (melting point 193 C.) are obtained. When using isodurol, the melting point of the bis-chloromethyl-compound is 107 C.

90 parts of bis-chloromethyl-tetra-methyl-benzene are oxidized with nitric acid in two stages as indicated in Example VII. After evaporating the nitric acid, 105 parts of mellitic acid are obtained which may, if desired, be re-crystallized from concentrated nitric acid.

Example IX 100 parts of p-chloromethyltoluene are heated to boiling for two hours at normal pressure in a vessel with a ceramic covering with 850 parts of 32% nitric acid, with simultaneous introduction of air. 90 parts of crude tolu'ic acid with 12% of chlorine are obtained. Its acid number is about 370. I

The crude acid is washed with toluene, whereby chlorine-free toluic acid is obtained having an acid number of 412. It is oxidized with 600 parts of 32% nitric acid for two hours at 160 C. and 5 atmospheres pres- Example X parts of pure 100 parts of p-chloromethyltoluene are heated for two hours at normal pressure at 105-110 C. with 900 parts of 45% nitric acid with the introduction of air. 97 parts of toluic acid are obtained in nitric acid solution, which is substantially free of hydrochloric acid. The hot emulsion is pumped into a stainless steel reactor and oxidized for one hour at 170 C. and 10 atmospheres of pressure, air being passed in simultaneously. Crude terephthalic acid with an acid number of 660 is obtained. This acid gives parts of terephthalic acid with an acid number of 673 after washing with methanol.

By carrying out the oxidation without the addition of air, only 80 parts of terephthalic acid are obtained and 400 parts of nitric acid are lost, whereas only 20 parts of nitric acid are lost with simultaneous introduction of air. The pressure must be raised to 15-20 atmospheres and the temperature to 180-200 C.

Example XI 100 parts of p-chloromethyltoluene are heated for two hours at -110 C. with 900 parts of 45 nitric acid with the introduction of oxygen. After separation and washing of the crude acid, 97 parts of chlorine-free toluic acid are obtained. The toluic acid is admixed with 32% nitric acid and pumped into a stainless steel reactor. After oxidizing for one hour at C. and 10 atmospheres, crude terephthalic acid with an acid number of 665 is obtained. This acid yields 90 parts of terephthalic acid with an acid number of 674 after washing with methanol.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A method of producing benzene polycarboxylic acids, comprising the steps of heating a benzene derivative consisting of a benzene ring substituted with at least one saturated alkyl group containing less than 3 carbon atoms and with at least one chloromethyl group with nitric acid at substantially atmospheric pressure at a temperature sufiiciently high to oxidize said chloromethyl group until a substantially chlorine-free reaction product is formed, thereby converting said chloromethyl group to a carboxyl group without substantially affecting said alkyl group and thus forming an alkyl benzene carboxylic acid; and heating said chlorine-free reaction product with nitric acid under a superatrnospheric pressure of 3-30 atmospheres at a temperature sufiiciently high to oxidize said alkyl group, thereby converting said alkyl group to a carboxyl group and thus obtaining a benzene carboxylic acid consisting of a benzene ring substituted only with carboxyl groups. p

2. A method according to claim 1 in which said benzene derivative is bis-chloromethyl xylene and said obtained benzene carboxylic acid is pyromellitic acid.

3. A method according to claim 1 in which said benzene derivative is p-chloromethyltoluene and said obtained benzene carboxylic acid is terephthalic acid.

4. A methodaccording to claim 1 in which said benzene derivative is bis-chloromethyl pseudocumene and said obtained benzene carboxylic acid is benzene-pentacarboxylic acid.

5. A method according to claim 1 in which said benzene derivative is bis-chloromethyl durene and said obtained benzene carboxylic acid is mellitic acid.

6. A method of producing benzene polycarboxylic acids, comprising the steps of heating a benzene derivative consisting of a benzene ring substituted with at least one saturated alkyl group containing less than 3 carbon atoms and at least one chloromethyl group, the total number of said alkyl and chloromethyl groups being at least three, with nitric acid of 30-47% concentration at substantially atmospheric pressure at a temperature sufiieiently high to oxidize said chloromethyl group until a substantially chlorine-free reaction product is formed, thereby converting all chloromethyl groups to carboxyl groups without substantially affecting said alkyl groups and thus forming an alkyl benzene carboxylic acid; and heating said chlorine-free reaction product with nitric acid of 20-45% concentration under a super-atmospheric pressure of 3-30 atmospheres at a temperature sufficient- 1y high to oxidize said alkyl groups, thereby converting said alkyl groups to carboxyl groups and thus obtaining a benzene carboxylic acid consisting of a benzene ring substituted only with carboxyl groups.

7. A method of producing benzene polycarboxylic acids, comprising the steps of heating a benzene derivative consisting of a benzene ring substituted with at least one saturated alkyl group containing less than 3 carbon atoms and with at least one chloromethyl group with nitric acid at substantially atmospheric pressure at a temperature corresponding substantially to the boiling range of said nitric acid so as to oxidize said chloromethyl group until a substantially chlorine-free reaction product is formed, thereby converting said chloromethyl group to a carboxyl group without substantially affecting said alkyl group and thus forming an alkyl benzene carboxylic acid; and heating said chlorine-free reaction product with nitric acid under a superatmospheric pres sure of 3-30 atmospheres at a temperature of about 150- 200 C. so as to oxidize said alkyl group, thereby converting said alkyl group to a carboxyl group and thus obtaining a benzene carboxylic acid consisting of a benzene ring substituted only with carboxyl groups.

8. A method of producing benzene polycarboxylic acids, comprising the steps of heating a benzene derivative consisting of a benzene ring substituted with at least one saturated alkyl group containing less than 3 carbon atoms and with at least one chloromethyl group with nitric acid of between 30-47% concentration at substantially atmospheric pressure at about the boiling point of the reaction mixture at atmospheric pressure so as to oxidize said chloromethyl group until a substantially chlorine-free reaction product is formed, thereby converting said chloromethyl group to a carboxyl group without substantially affecting said alkyl group and thus forming an alkyl benzene carboxylic acid; and heating said chlorine-free reaction product with nitric acid of between 14 20-45% concentration under a superatmospheric pressure of 3-30 atmospheres at a temperature of about ISO-200 C. so as to oxidize said alkyl group, thereby converting said alkyl group to a carboxyl group and thus obtaining a benzene carboxylic acid consisting of a benzene ring substituted only with carboxyl groups.

9.-A method of producing benzen'e polycarboxylic acids, comprising the steps of heating a benzene derivativeiconsisting of a benzene ring substituted with at least one saturated alkyl group containing less than 3 carbon atoms and with at least one chloromethyl group with nitric acid at substantially atmospheric pressure at a temperature sufficiently high to oxidize said chloromethyl group while bubbling a gas containing free oxygen through the reaction mixture until a substantially chlorine-free reaction product is formed, thereby converting said chloromethyl group to a carboxyl group without substantially aifecting said alkyl group and thus forming an alkyl benzene carboxylic acid and evolving a gas including nitrogen oxide; and heating said chlorine-free reaction product with nitric acid under a superatmospheric pressure of 3-30 atmospheres at a temperature sufiiciently high to oxidize said alkyl group while bubbling a gas containing free oxygen through the reaction mixture, thereby converting said alkyl group to a carboxyl group and thus obtaining a benzene carboxylic acid consisting of a benzene ring substituted only with carboxyl groups and evolving a gas including nitrogen oxide.

10. A method of producing benzene polycarboxylic acids, comprising the steps of heating mono-chloromethyl xylene and nitric acid at substantially atmospheric pressure at a temperature sufiiciently high to oxidize the chloromethyl group of said mono-chloromethyl xylene to a carboxyl group until a substantially chlorine-free reaction product is formed, thereby converting said chloromethyl group to a carboxyl group without substantially affecting the alkyl group of said mono-chloromethyl xylene, thus forming dimethyl-benzene mono-carboxylic acid; and heating said dimethyl-benzene mono-carboxylic acid with nitric acid of 20-45% concentration under a superatmospheric pressure of 3-30 atmospheres at a temperature sufficiently high to oxidize said methyl groups, thereby converting said methyl groups to carboxyl groups, thus otbaining trimellitic acid.

References Cited in the file of this patent UNITED STATES PATENTS 1,332,028 Coblentz et al. Feb. 24, 1920 1,576,999 Seydel Mar. 16, 1926 2,636,899 Burrows et al. Apr. 28, 1953 2,740,811 Lotz Apr. 3, 1956 FOREIGN PATENTS 494,439 Belgium Mar. 31, 1950 517,238 Belgium Feb. 14, 1953 698,734 Great Britain Oct. 21, 1953

Claims (1)

1. A METHOD OF PRODUCING BENZENE POLYCARBOXYLIC ACIDS, COMPRISING THE STEPS OF HEATING A BENZENE DERIVATIVE CONSISTING OF A BENZENE RING SUBSTITUTED WITH AT LEAST ONE SATURATED ALKYL GROUP CONTAINING LESS THAN 3 CARBON ATOMS AND WITH AT LEAST ONE CHLOROMETHYL GROUP WITH NITRIC ACID AT SUBSTANTIALLY ATMOSPHERIC PRESSURE AT A TEMPERATURE SUFFICIENTLY HIGH TO OXIDIZE SAID CHLOROMETHYL GROUP UNTIL A SUBSTANTIALLY CHLORINE-FREE REACTION PRODUCT IS FORMED, THEREBY CONVERTING SAID CHLOROMETHYL GROUP TO A CARBOXYL GROUP WITHOUT SUBSTANTIALLY AFFECTING SAID ALKYL GROUP AND THUS FORMING AN ALKYL BENZENE CARBOLIC ACID, AND HEATING SAID CHLORINE-FREE REACTION PRODUCT WITH NITRIC ACID UNDER A SUPERATOMSPHERIC PRESSURE OF 3-30 ATMOSPHERES AT A TEMPERATURE SUFFICIENTLY HIGH TO OXIDIZE SAID ALKYL GROUP, THEREBY CONVERTING SAID ALKYL GROUP TO A CARBOXYL GROUP AND THUS OBTAINING A BENZENE CARBOXYLIC ACID CONSISTING OF A BENZENE RING SUBSTITUTED ONLY WITH CARBOXYL GROUPS.